PAPERS ON CHEMISTRY AND PHYSICS 303 



to be calculated in atmospheres. This constant is found 

 by dividing the observed slope of the log P vs 1/T cur^^es 

 by the absolute boiling point. By adding the value of the 

 logarithm of 760, which is practically 2,8S1, to the num- 

 bers given in column 13 there is obtained the value of the 

 constant to be used in order to represent the vapor pres- 

 sure in mm. of mercury. These values are given in 

 column 14. This constant increases as the boiling point 

 of the substance in question increases. 



In order to show the limitations of this method of cal- 

 culating vapor pressures and heats of evaporation, there 

 are collected at the bottom of Table I the data for some 

 typical associated liquids. In general it may be said 

 that the simpler compounds containing hydroxyl, amino, 

 carbonyl and carboxyl groups and most molten salts will 

 deviate more or less from the general rule for normal 

 liquids. For these classes of liquids there will be needed 

 at least two values of the vapor pressure or one value 

 of the vapor pressure and the latent heat of evaporation 

 in order to write the vapor pressure equation. It should 

 be noted also that the vapor pressures of the more 

 strongly associated substances may not be accurately 

 represented by the straight line equation except through 

 relatively narrow ranges of temperature. 



Having given a simple expression for the vapor pres- 

 sure of liquids, the question may be asked, Can a similar 

 expression be derived for the sublimation pressure of 

 solids "? Happily the answer is that a similar expression 

 exists, and that for many substances the constants may 

 "be empirically calculated from existing data. 



It has been shoA^^i that in the expression. 

 Log p = c — S/T (8) 



the constants C and S may be calculated for normal 

 liquids from the boiling point alone. In any case they 

 may be calculated from two simultaneous values for log 

 P and T. Xow it may be shown that there is an exactly 

 similar expression for sublimation pressure, viz., 



L,og Ps = C. — Ss T (9) 



in which Ps is the sublimation pressure at the tempera- 



